Character

Sanemi has bloodshot eyes, looks very ferocious, and has scars all over his face and body. Sanemi is abrasive, hot-blooded, and stubborn, oftentimes impulsive and quick to lash out. In Tanjiro’s words, he seems to know nothing about reasoning and intelligence.

Contrary to his appearance and his outrageous behavior, deep down he is a rational person.
He respects seniority and is highly self-disciplined. Besides he is highly cooperative and can get along with other Hashiras.
In the presence of the beloved master Kagaya Ubuyashiki, he displays the highest reverence and speaks intelligently and courteously.

Though he respects the Master very much, he fails to restrain his uncommon hatred for demons. Ignited by this hatred he slashed his arm to bait Nezuko into attacking him. Even after Nezuko held herself and didn’t attack as Sanemi expects, and Kagaya used this as a proof that Nezuko wouldn't attack humans, Sanemi didn’t truly accept the Kamado kids.
Since then he cannot get along with Tanjiro. Even after the Kamado kids have proved themselves by making great contributions to the Demon Slayer Corps, he still sticks to his opinion and refuses to accept them.

He is also the old brother of Genya Shinazugawa, who joins Demon Slayer Corps around the same time as Tanjiro. However, he stubbornly refuses to claim kin with Genya and keeps pushing away Genya all the while.

Ability

Wind Breathing is one of the basic five full focus breathing styles. And demon slayers use full focus breathing styles to decapitate demons. By using this breathing style, the swordsman dashes forward a cyclone of slashes that wield down on the enemies like a Kamaitachi.
Until now, seven styles of the Wind Breathing have been disclosed.
For detailed information, please refer to related articles.

Fighting Style

Born with a ferocious temper, combined with those experiences of reckless battles against demons before he joins Demon Slayer Corps, Sanemi is good at dealing with all changes in the battlefield, including terrains and conditions. He is even able to capitalize on those changes. Besides, He has been using his blood tactics in fighting for a long time. As a result, he develops a high tolerance to pains which assures his quick restoration of mental and physical conditions.

Equipment

As a demon slayer, he is furnished with uniform and Nichirin Sword.
Sanemi opens the front of the uniform wide. And on top of the uniform, he wears a white haori that has a kanji character meaning “KILL” (killing all the demons) on the back.

The Nichirin swords are forged from a unique scarlet ore which is found on Sunlight Mountain and can constantly absorb sunlight all year round. Decapitation by Nichirin sword is the only way a human can kill an immortal demon. Hashiras’ Nichirin swords are engraved with four kanji characters meaning “DESTROY DEMONS”.

Nichirin swords change colors depending upon the user, making each blade unique. Sanemi’s sword is dark green implying that he is highly compatible with the Wind Breathing style.
The windmill-like hand-guard of his sword is made of eight rhombus components. Also, the blade has a billowing wind quench pattern that is rarely seen on a Nichirin sword. Usually, a Nichirin sword would place stress on practicality instead of appearance. The scabbard is black and covered with harsh scratches. It is not clear if those harsh scratches are designs or result from frequent battles.

History

Sanemi was the eldest son of seven children and lived with both of his parents. Except for their father, other family members were very close.
Their father was abusive, constantly lashing out at the children and their mother until one day, he was stabbed and killed by someone with a grudge against him. Sanemi and Genya thought their father just deserved it and resolved to protect their mother and the family in future days.

One day, Sanemi's mother failed to return home, so Sanemi left to search for her. During Sanemi’s absence, Genya and other siblings were slaughtered by a dark unknown monster. Later, Sanime returned and tackled the monster through the window to the street. Then, they got into fierce fighting. Sadly enough, the dark unknown monster was actually their mother who was turned into a demon. The next day morning, after the sun raised, Genya ran into the street to find a doctor, then he met Sanemi on the street.
Sanemi, spattered with blood, was standing rooted to the spot. And their mother was lying down on the ground. Shocked by this scene, Genya screamed and accused Sanemi of「murdering」 their mother. Finally, in Genya’s arms, their mother’s body dissolved in the sunlight.
(Those experiences may be the reason why Sanemi showed the strongest malice towards Nezuko and Tanjiro during the Hashira sanction. And, he could not get along with Tanjiro, because Tanjiro once said he should quit being a Hashira if he was unable to tell a good demon from a bad one.)
Genya and Sanemi lost everything overnight. Five of their siblings actually died immediately after being attacked. (Those harsh scars on their faces were lacerated that night)

It was not clear how the brothers sorted this out later on. However, after this, a great estrangement developed between Genya and Sanemi. Sanemi started to declare that 「I don’t have any little brother」. When Genya visited him in Demon Slayer Corps, he treated Genya coldly, pushed him off and insisted that 「You stupid asshole are not my little brother! You should just quit the idea of joining Demon Slayer Corps 」.
On the other hand, Genya thought that「I want to apologize to brother for the words I said, and want to be accepted」. So, he constantly felt insecure as he wanted to become a Hashira like his brother as soon as possible.

In his obstinate refusal to accept Nezuko even though three people have sworn on their lives that Nezuko would never devour humans, we can know that his mother’s death still lingers on.

Sanemi Shinazugawa’s true nature（spoiler alert！！！）

The true nature

「……I really don’t know how to do with you」「I even killed our mother to protect you. Do you know why?」

In the all-out confrontation in Muzan’s stronghold: the Dimensional Infinity Fortress, Muichiro and Genya confronted the strongest one of the Twelve Demon Moons-Upper Moon One Kokushibo. Sanemi arrived at the battlefield when Muichiro and Genya were about to be killed.
On this occasion, Sanemi finally confessed to Genya about why he treated Genya so bad, and why he stayed away from Genya. The truth was that 「I just want you to build a family of your own somewhere, and share the dreams of our mother, brother and sister to live a life as happy as you can!」. It was for the same reason that he kept chasing Genya out of Demon Slayer Corps for. It was the so-called the other side of love.
However, Tanjiro seemed to get to understand Sanemi’s thoughts by instinct. During the Hashira training, when he met Genya in Himejima’s training course, he told Genya that though Sanemi was「very angry about Genya’s joining Demon Slayer Corps」, 「there was not the slightest sign of detestation」; and he believed that 「Sanemi cares for Genya deeply all the time 」.

As it is mentioned above, Sanemi has an unusual obsession with killing demons. It is because he cannot let his departed fellow fighters die for nothing, and he is devoted to creating a world where his only family-his beloved little brother can live a happy life. Driven by this strong faith, Sanemi grows to what he is now.
So after he made his confession, he shouted to Kokushibo that 「How dare you slash my brother you shitty bastard! I’ll never let you get away with it」. It can be said that Sanemi’s tension at this moment was most agitated ever.

Spoiler alert

「Demons will get drunk at the smell of my blood. Mine is the rarest blood of all. Come on, taste it to your heart’s content！！」

In chapter 167, Sanemi’s blood type is revealed to be the marechi (rare blood) which can intoxicate demons.
Moreover, his blood type is 「the rarest of all marechi blood」.
Demons would act intoxicated and even get unable to stand up at the smell of marechi blood. Therefore, a bleeding injury can be an advantage to Sanemi.

（For this fact, Nezuko who has been forced to smell his blood at a very close distance self-proves her mental toughness. Also for this fact, Sanemi himself has proved that demon Nezuko will not attack humans though he does not plan to）

As a Hashira, the number of his scars is unusually high. As long as demons can be destroyed, he never hesitates to cut himself for blood. Besides, though it’s not described in detail, from Mitsuri’s words 「got more attractive with that new scars」, we can see that Sanemi still cuts himself for blood every time on his own initiative.

His demonized mother suddenly became sluggish the moment he started to bleed. He then realized that his blood might be special. After losing his family, without knowledge of Demon Slayers nor Nichirin Blades, Sanemi fought demons at night, incapacitating them with his blood, then stringing them up to let the morning sun kill them. He used this crude and reckless battle style to kill demons for quite a while.
While wandering the land hunting demons, he eventually met a legitimate Demon Slayer, Masachika Kumeno, who helped him and introduced him to a proper trainer in Demon Slayer Corps.
As it is mentioned above, Sanemi though is abrasive in words, deep down he is actually affectionate. Especially, he relies on Kumeno who treats him like a brother. However, Kumeno later lost his life in a battle against demons.
Therefore, after being promoted to the rank of Wind Hashira, bitter over Kumeno's death, Sanemi takes his anger out on『the Master』, accusing him of happily giving orders from the safe place while the Demon Slayers bleed and die for his cause. He acts offensively every time talking to the Master, which incurs other Hashiras’ displeasure.
Kanae then informs him that Ubuyashiki has memorized the names and backgrounds of every single demon slayer who has died since he became leader. This discovery, and reading Kumeno's will given to him by Ubuyashiki, gives Sanemi newfound respect for him as the leader of the Demon Slayer Corps.

Kimetsu Gakuen

In volume 18, in the self-parody Kimetsu Gakuen, Hashiras who have reached their adulthood become teachers. And Sanemi is a math teacher.
He leaves his collar widely open all year round, even when attending weddings and funerals. Otherwise, he would feel stressful.
He talks to Amane who is also a teacher a lot. Students will be thrown out from the window by Sanemi, if they declare that 「math is useless」 in front of him. He is very kind to old people and girls. However, every baby cries if he comes close.

In the Kimetsu Gakuen world, his little brother Genya joined the school’s shooting club and became the club’s leading player for winning many awards worldwide.
However, Genya is bad at mathematics though his brother is a math teacher, which causes Sanemi’s strong dissatisfaction. On one occasion when Genya was receiving an award, Sanemi suddenly appeared and tore Genya’s certificate, shouting 「if you have time for this, word harder on math」. At this time, Genya, overwhelmed by Sanemi’s moral harassment, could do nothing but stand trembling on the stage. Genya’s classmates who used to fear Genya develop a soft place in hearts for him after watching Sanemi’s moral harassment. In the end, Genya seems to gain a few more friends.
Many fans declare that they are completely shocked by the totally different brotherhood from the main storyline.

Digression

In the character book, the author calls his open-chest dressing style the「erotic」. So many readers tend to also call him the『Erotic Hashira』.
Moreover, the author also writes something like:「he must be very proud of his muscle」. All of these trigger more naming from readers, such as 『Succu Hashira』『a walking aphrodisiac』
Someone goes even further, calling Sanemi 『prurience』,『sea otter potfor demons』, etc.
（Attention. There are also a lot of people who hate these jokes）

The little brother Genya’s physical condition is also very special. And he also utilizes his physical condition to fight. There is a hypothesis among readers that 「maybe it is the special physical condition that makes the Shinazugawa family targeted by demons」.